1. Field of the Invention
This invention relates to a car exhaust gas purification device controlling circuit for automatically controlling the air-fuel ratio of carburetor type gasoline engines.
2. Description of the Prior Art
Generally, the amount of car exhaust gas pollutants including CO, HC and NO.sub.x is determined definitely by the air-fuel ratio. Accordingly, in view of the fact that poison-free treatment of the NO.sub.x gas is difficult, as general approach to solve this problem, there have been proposed various counter measures which are effective in the low generation density region of NO.sub.x gas i.e., thick mixture or thin mixture region, these countermeasures including improvements for fuel cost and operation efficiency. According to these conventional countermeasures, however, there still remain problems such as secondary pollution due to catalizers, degradation of operation efficiency due to decrease in output power and increase in the fuel cost, leaving behind many problems to be discussed for these countermeasures to be practised.
For the background as above, a gas purification system of new type is demanded which is capable of simultaneously decreasing the amount of NO.sub.x, CO and HC without giving adverse affect upon the fuel cost and operation efficiency. In the circumstances, a system utilizing a ternary catalizer is highlighted as a promising measure, where the ternary catalizer is a single catalizer which can oxidize CO and HC and at the same time reduce NO.sub.x.
FIG. 1 shows ternary characteristics of the ternary catalizer, where abscissa represents air-fuel ratio A/F and ordinate purification rate .eta. of the ternary catalizer. Purification rates .eta.NO.sub.x, .eta.CO and .eta.HC for respective pollutants NO.sub.x, CO and HC have characteristics as shown in the figure. As will be seen from the characteristics, respective purification rates .eta.NO.sub.x, .eta.CO and .eta.HC are more than 90% at a theoretical air-fuel ratio and in proximity thereof. The theoretical air-fuel ratio referred to herein has the meaning of a weight ratio between a unity of fuel and a quantity of air necessary and sufficient for flaming the unitary fuel and it amounts to about 14.7. It will also be appreciated from FIG. 1 that the proximity of the theoretical air-fuel ratio lies within .+-.0.1. Accordingly, it is expected that the use of ternary catalizer offers a purification system of high performance.
From the above point of view, some systems have conventionally been proposed, of which a typical example is illustrated in FIG. 2. In accordance with this example, a ternary catalizer 3 is placed in an exhaust conduit 2 communicated with an engine 1, a gas sensor 4 for detecting one composition of exhaust gas, for example oxygen, is mounted to a portion of the exhaust conduit 2 upstream of the ternary catalizer, and the output of the gas sensor 4 is fed via a controlling circuit 5 to an electromagnetic valve 7 provided for a carburetor 6 to drive the valve 7, whereby the air-fuel ratio in the carburetor is controlled to the theoretical air-fuel ratio and proximity thereof at which the ternary catalizer reacts most efficiently. Needless to say, fuel is delivered to the electromagnetic valve 7 and air to the carburetor 6. In such a system, however, since the output of the gas sensor 4 eventually drives the electromagnetic valve 7 to vary an opening area A of an air bleed of the carburetor 6 thereby to control the air-fuel ratio, the operation takes the form of a proportional controlling with the result that the controlled air-fuel ratio is accompanied with an adverse offset as will be described later.
FIG. 3 is a graph showing occurrence of an offset in the air-fuel ratio to be controlled, where the abscissa represents controlling current I.sub.C for the electromagnetic valve and ordinates represent air-fuel ratio A/F, electromagnetic valve stroke S and air bleed opening area A. Air-fuel ratio A/F depends on a positional relationship between the stroke of the proportional electromagnetic valve and the air bleed but has a hysteresis width D as shown in FIG. 3 on account of the negative suction pressure and magnetic hysteresis of the electromagnetic valve. Consequently, for a controlling current I.sub.C of 0.4 amperes, for example, the air-fuel ratio drifts within 13.6 to 15.4, leading to positive and negative offsets relative to the theoretical air-fuel ratio of 14.7.
A similar technique to the conventional example as above is disclosed in the specification of U.S. Pat. Nos. 3,738,341 and 2,355,090. Systems disclosed in these patents have substantially the same construction as the above conventional example except that correction in view of oxygen concentration and acceleration are taken into consideration. Accordingly, these patents are disadvantageous, like the conventional example quoted hereinbefore, in that a complete controlling of the air-fuel ratio is not accomplished at the theoretical air-fuel ratio and in proximity thereof.